There are various competing theories why a pound sterling is called a "quid," and no one is completely certain which one is true. The most-common explanation is that the term comes from Latin.
The Latin phrase "quid pro quo" means "what for what," or figuratively, "an equal exchange." Another theory is that "quid" is a modified version of the Italian word "scudo," which was the name of several Italian coins. The term "quid" existed before Britain converted to a decimal system in 1971. In fact, it could have been in usage as early as the first century, when Romans occupied Britain. The Romans spoke Latin, and thus, the term may have come to exist during this period.